1. Field of the Invention
The present invention relates to star-shaped zinc oxide particles excellent in ultraviolet-ray absorbability, and to a method for efficiently producing the star-shaped zinc oxide particles through a simple process without any specific apparatus.
2. Description of the Related Art
Zinc oxide particles are used in various applications such as white pigments, ultraviolet-ray shielding agents, fillers, adsorbents, photocatalysts, catalysts, ceramics raw materials, conductive agents, piezoelectric materials, gas sensors, electrophotographic photosensitive materials, varistors, phosphors, emitters and electronic devices. In addition, they are used in, for example, cosmetics, external preparations and resin compositions. In particular, zinc oxide particles are widely used as an ultraviolet-ray shielding agent contained in sunscreen cosmetics, since they are excellent in ability to shield rays of the ultraviolet A (UVA) region (wavelength: 320 nm to 380 nm) and in transparency with respect to visible light.
As to such zinc oxide particles (their production method), for example, flower-shaped (carnation-shaped) zinc oxide aggregates in which primary particles having an average particle diameter of 50 nm to 100 nm are planarily assembled are proposed in Japanese Patent (JP-B) No. 4030133 and “INDUSTRIAL MATERIAL, 53 (2) 60-63 (2005).” However, such zinc oxide aggregates that have been proposed above are particles' aggregates each of which does not possess a structure in which the particles are accumulated and grown in a branched manner, the particles being accumulated so that crystallographic orientations thereof are aligned in a same direction. Also, production of the flower-shaped (carnation-shaped) zinc oxide aggregates requires a Double-Jet reaction apparatus. This is because they are produced by firing basic zinc carbonate in an aqueous reaction solution which is formed as follows. Specifically, an aqueous reaction solution of zinc ion (Zn2+), carbonate ion (CO32−) and hydroxide ion (OH−) in water (reaction solvent) is allowed to react while the pH of the aqueous reaction solution is being maintained to be 7 to 9 and the mole ratio of the hydroxide ion to the carbonate ion (1) is being adjusted to 2.5 to 3.5. In addition, “Chem. Mater. 15 (16) 3202-3207 (2003)” describes that star-shaped zinc oxide particles are produced by accumulating nano-sized crystallites having a size of 16 nm to 33 nm so that their crystallographic orientations are aligned in a same direction. But, according to the method described in this literature, a complicated process must be performed using a Double-Jet reaction apparatus in order to produce star-shaped zinc oxide particles.